Active matrix light emitting diode displays offer many potential advantages when compared to active matrix liquid crystal displays. Some advantages include, but are not limited to, superior image quality, thin profile, low power consumption, and lower cost.
Currently, two different methods are used in addressing active matrix liquid crystal displays; namely, voltage programming and current programming. A voltage programming method benefits from a large installed base of display drivers that operate in a voltage programming mode. However, voltage programmed pixel circuits suffer from the lack of ability to compensate for the variations in the pixel TFT drive currents across the surface of the display, which leads to luminance non-uniformities in the display. A current-programming method may compensate for the variations in the drive TFT performance across the display surface, which results in better display luminance and color uniformity than voltage-programmed pixels. For these reasons, current-programmed pixels are preferred over voltage-programmed pixels.
Notwithstanding the above-referenced preference, one drawback to current-programmed LED displays is that they exhibit longer pixel programming times than voltage-programmed pixels, particularly for lower gray levels. Longer pixel programming times are caused because current-programmed displays typically use small programming currents (e.g., 7.8 nA to 2 μA) for a typical 8-bit display driver with an 80 color groups per inch (CGPI) resolution, or even smaller currents for smaller pixel sizes in higher resolution displays. One reason for the prolonged programming time is that the data bus capacitances need to be charged before the pixel can be properly programmed, and it takes a significant amount of time to charge the data bus capacitances with these small amounts of programming current, as the data bus capacitance is significantly larger than the pixel capacitance. To alleviate this problem of slow pixel data programming times in current mode column drivers, voltage pre-charging methods have been developed as described in U.S. Pat. Nos. 7,012,378 and 7,167,406. U.S. Pat. No. 7,012,378 addresses the problem by sequentially (as the rows are scanned) applying a fixed DC pre-charge voltage to the data buses in the display during a short pre-charge interval, and then applying current programming to the pixels. The DC voltage pre-charge improves current-programmed pixel operation at low luminance (low programming currents); however, this fixed DC pre-charge voltage is useful for a very restricted range of display brightness levels (gray levels), as very low brightness levels (gray levels) require a different DC pre-charge voltage than very high brightness levels. U.S. Pat. No. 7,167,406, on the other hand, expands the pre-charge voltage's utility by providing a pre-charge voltage proportional to the desired pixel programming current; however, there are still significant shortcomings to the method described in U.S. Pat. No. 7,167,406. One shortcoming is that the use of a proportional DC pre-charge voltage does not result in sufficient display color and luminance uniformity due to the drive requirements for a red, green, and blue (R, G, B) LED pixel being different, and the pixel current feed-through effects. The pixel feed through current is a consequence of the pixel TFT switching at the end of the programming time, which may result in increasing or decreasing the current through the LED from the programmed value by ΔIP. This phenomenon produces a pixel luminance which is lower than the desired pixel luminance, and the value of ΔIP depends upon the pixel gray level and the parasitic capacitance of the drive TFT.
The present invention substantially improves upon the prior art, and provides operational flexibility not provided by the prior art for achieving uniform color and gray level luminance in active matrix light emitting diode displays. The present invention integrates voltage pre-charge circuitry within the current-programmed column driver, and provides novel and practical means to optimize current-programmed pixel operation to achieve superior color and gray level luminance uniformity in the display. The present invention also provides programmable, non-proportional lookup tables to establish and define unique and optimum voltage pre-charge levels, and programming currents for each desired pixel color and luminance level (pixel gray level) by including compensation for the differences in R, G, B LED pixel drive requirements and current feed-through effects at the end of the pixel programming time.
Accordingly, it is desirable to provide drivers, displays, and methods for controlling the luminance of the LEDs in a display by decreasing the amount of time needed to charge the data bus capacitances. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.